In some printers, a roller for retaining paper is mounted such that it is rotatable about a shaft and that it can be slid axially when subjected to a force greater than a given magnitude. The roller is shifted in the axial direction according to the width of the paper. A known structure for effecting such an operation is shown in FIG. 3, in which a roller b is centrally provided with a hole c so as to be loosely and rotatably inserted on a shaft a. A guide groove d is formed in the inner surface of the hole c, and a ring spring e shaped like the letter "C" is inserted in the groove d. The spring e is resiliently urged against the outer periphery of the shaft a. The roller b can be rotated at that axial position about the shaft a while guided by the spring e. The roller b can be slid axially by pushing it with a force greater than the retaining force exerted by the spring e on the shaft a. In this conventional structure, it is difficult to insert the ring spring e in the guide groove d during assembly of the structure. Further, it is difficult to form the roller b by injection molding because of the presence of the groove d. Accordingly, the groove d is formed by machining and hence the structure is manufactured and assembled quite inefficiently.